Coriolis flowmeters operate on the Coriolis Effect. A mass flow dependent Coriolis force occurs when a moving mass is subjected to an oscillation perpendicular to the flow direction. Coriolis flowmeters generally comprise at least one measurement tube and a driver for setting the tube into an oscillatory motion. In operation, the fluid flows through the oscillating tube. One type of driving mechanism is an electromechanical driver that imparts a force proportional to an applied excitation signal, i.e. a current or a voltage. Measurement tube and fluid form an oscillatory system that is normally operated at a resonance frequency. The resonance frequency depends on the material of the measurement tube and a density of the fluid. The Coriolis force is induced by the oscillatory motion. A Coriolis reaction force experienced by the traveling fluid mass is transferred to the measurement tube itself and is manifested in a change of motion of the tube. Two motion sensors detect this change. Mass flow is usually determined based on a phase difference between the measurement signals derived by the motion sensors.
In order to operate the flowmeter in a resonant mode of vibration, most flowmeters comprise a feed back loop generating the excitation signal based on the measurement signals of the motion sensors. Feeding back the sensor signal to the driver permits the drive frequency to migrate to the resonant frequency.
Coriolis flowmeters are used in applications where repeatable and stable measurement of liquid mass or volume flow is required. In applications where successive batches of products are processed conventional flowmeters can suffer from poor meter accuracy and repeatability. The reason for this is that whenever a sudden transition from a full measurement tube to an empty measurement tube or vice versa occurs, the physical properties of the oscillatory system change dramatically and the flow meter needs time to adjust in order to establish a desired mode, frequency and amplitude of the motion for the respective measurement situation. During this transition time, conventional coriolis flow measurement based on the measurement signals derived by the motion sensors do not produce accurate results. Erroneous flow spiking or outputs during no-flow situations are common operational problems.
Currently, mass flow meter manufacturers approach solutions to overcome this problem in four ways:                1) The meter exceeds a manufacturer predefined limit of excitation signal or drive gain and faults, locking the output variables at a fail-safe condition.        2) The meter has two separate drive circuits, evaluates the drive gain from normal conditions, and compensates for this change, while applying a mathematical algorithm adjusting the measured mass flow variable.        3) A density limit is programmed into the meter to account for the detected change in the measured product density, as a means to regulate a fail-safe point.        4) A low flow cut-off is elevated.        
Unfortunately, these four methods result in measurement accuracy and performance issues:                A) The predefined fault limit may be significantly higher than the actual process condition which produces an erroneous flow output or the meter output locks up, requiring frequent manual intervention,        B) Process evaluation of two drive circuit gain levels can be falsely interpreted and the resulting measuring uncertainty exceeds 2%.        C) The density limit point or range is slower to react to the change, including reaction to empty measurement tube conditions.        D) Elevation of the low flow cut-off can miss measurable product flow or is ineffective as a recognition technique.        
Some of these approaches are for example described in U.S. Pat. No. B2 6,505,519. In this patent, a self-validating meter is described, which provides a best estimate of a parameter to be measured, e.g. mass flow, based on all information available This flowmeter comprises a controller, which derives a raw measurement value based on the sensor signals of the motion sensors. When the controller detects no abnormalities, the controller has nominal confidence in the raw measurement value and produces a validated measurement value equal to the raw measurement value. When the controller detects an abnormality in the sensor, it produces a validated measurement value, which is a value that the controller considers to be a better estimate of the parameter to be measured.